For certain operations and manufacturing processes, pneumatic actuators are used to perform pressing functions in which part of the pressing stroke, known as the approach stroke or the idle stroke, is completed without encountering any specific resistance, while the remainder of the stroke, described hereafter as the active stroke, requires a particular predetermined thrust force.
A specific case of this kind is that of internal mixers for rubber and elastomers, in which a pressing weight has to be raised by a corresponding pneumatic actuator, to make it possible to open the loading hatch for the material to be processed, and then lowered until it comes into contact with the loaded material and then presses it into the mixing chamber with a predetermined degree of pressure to permit complete processing of all the material. In this case a large amount of compressed air, supplied to the upper chamber of the actuator, is required to complete a stroke, part of which is necessary only to lower the weight connected to the actuator after it has been raised to open the access hatch of the mixing chamber, while the part of the stroke in which active pressure is required is limited. The necessity of supplying large volumes of compressed air entails high costs for the pneumatic equipment and high energy consumption by this equipment.